This invention relates to a method of forming an internal annular groove in a plastic tube part by pushing the tube part onto a core, surrounding same with a mould having an annular recess and by forming the groove by applying an expandable and retractable groove forming ring in the interior of the tube part which ring is removed by retracting same after forming the groove and cooling down the plastic tube part.
Such a method for forming an internal annular groove in a plastic tube part wherein the groove forming ring is expanded by moving groove forming segments outwardly, is known.
Since the outer diameter and the inner diameter of the plastic tube part will increase at the location of the groove, this automatically involves a decrease of the thickness of the wall of the tube part at said location. Though it has been tried to alleviate this disadvantage by surrounding the plastic tube part by a loose bush or sleeve which is deformed simultaneously with the plastic tube part, this method has the drawback that much work expenditure is involved in mounting said separate bush or sleeve, while moreover the produced plastic tube parts do not, in a great number of cases, comply with present requirements.